1. Field of the Invention
This invention relates to a method of producing a cooperating pair of punch and die used for pressing-forming a blank material into a panel product.
2. Description of the Related Art
Many parts of a motor vehicle, such as a fuel tank of a motorcycle, are produced by press working. In the press working, a press-forming die assembly formed from a precision-machined alloy steel is used in general. The alloy steel press-forming die assembly is highly durable and hence is able to recover its manufacturing cost when used in mass manufacturing systems of vehicle parts.
In recent years, model changes of motorcycles tend to occur at a relatively short cycle time, and the manufacturing system tends to shift from mass""manufacturing of one type of product to more flexible manufacturing of a wide variety of products with relatively low volume of production. The alloy steel press-forming die assembly, when used in flexible manufacturing systems, may sometimes fail to recover its manufacturing cost, leading to a cost-up of the final product.
With the foregoing problem in view, a press-forming die assembly formed from a synthetic resin is generally used for flexible manufacturing systems. The synthetic resin press-forming die assembly is relatively easy to manufacture and can be manufactured at a low cost as compared to the alloy steel press-forming die assembly. Accordingly, even when the model change cycle of the motorcycle is relatively short, the synthetic resin press-forming die assembly is able to recover its manufacturing cost and hence prevents a cost-up of the final product.
However, a working surfaces of the synthetic resin press-forming die assembly is softer than that of the alloy steel press-forming die assembly and hence is susceptible to wear in a relatively short period of use. Once wear occurs on the working surfaces, it is difficult to maintain the desired accuracies of the press-formed products. Accordingly, in order to maintain the desired product accuracies, the synthetic resin press-forming die assembly must be replaced before the working surface is worn away. With is requirement, the synthetic resin press-forming die assembly is replaced in a relatively short cycle and may sometimes fails to recover its manufacturing cost.
Another known relatively low-cost press-forming die assembly is made of a zinc alloy generally called xe2x80x9cZASxe2x80x9d. The ZAS is a zinc-based alloy containing aluminum and antimony and hence has a relatively low melting point such as about 380xc2x0 C. Because of this low molting point, the ZAS press-forming die assembly can be manufactured at a relatively low cost.
However, a working surface of the ZAS press-forming die assembly is relatively soft and hence is poor in durability as compared to that of the alloy steel press-forming die assembly. Due to a relatively short replacement cycle, the ZAS press-forming die assembly may sometime fail to recover its manufacturing cost.
There has been a keen demand for a press-forming die assembly which can be manufactured at a reduced cost and has high durability comparable to that of the alloy steel press-forming die assembly.
It is accordingly an object of the present invention to provide a method of producing a highly durable press-forming die assembly (punch and die) at a relatively low cost.
According to a first aspect of the present invention, there is provided a method of producing a press-forming die, comprising the steps of: preparing a master die and a master punch relatively movable toward and away from each other so as to define therebetween a die cavity; relatively reciprocating the master die and the master punch together while a first blank sheet is disposed between the master die and the master punch, thereby press-forming the first blank sheet into a press-formed panel complementary in contour to the shape of the die cavity; while a second blank sheet is disposed between the master die and the master punch, relatively reciprocating the master die and the master punch together with a stroke smaller than the stroke achieved when the press-formed panel is produced, thereby producing a half-pressed panel; applying an elastic material onto one surface of the half-pressed panel which has been situated in face to face with the master die; fitting the press-formed panel over the master punch and placing the half-pressed panel over the press-formed panel with the elastic member facing away from press-formed panel; forcing the half-pressed panel against the press-formed panel via the elastic material to thereby shape the half-pressed panel into a die panel complementary in contour to the shape of the press-formed panel; and applying a back-up material to the rear surface of the die panel to thereby produce a press-forming die.
According to another aspect of the present invention, there is provided a method of producing a press-forming punch, comprising the steps of: preparing a master die and a master punch relatively movable toward and away from each other so as to define therebetween a die cavity; relatively reciprocating the master die and the master punch together while a first blank sheet is disposed between the master die and the master punch, thereby press-forming the first blank sheet into a press-formed panel complementary in contour to the shape of the die cavity; while a second blank sheet is disposed between the master die and the master punch, relatively reciprocating the master die and the master punch together with a stroke smaller than the stroke achieved when the press-formed panel is produced, thereby producing a half-pressed panel; applying an elastic material onto one surface of the half-pressed panel which has been situated in face to face with the master punch; fitting the press-formed panel over the master die and placing the half-pressed panel over the press-formed panel with the elastic member facing away from press-formed panel; forcing the half-pressed panel against the press-formed panel via the elastic material to thereby shape the half-pressed panel into a punch panel complementary in contour to the shape of the press-formed panel; and applying a back-up material to the rear surface of the punch panel to thereby produce a press-forming punch.
It is preferable that between the forcing step and the applying step, the method comprises the additional step of applying a backing material to the rear surface of the die panel or the rear surface of the punch panel. The backing material comprises a water-soluble epoxy resin, and preferably a fibre-reinforced water-soluble epoxy resin.
The die panel or the punch panel is preferably made of steel, such as stainless steel or high-tensile steel, and has been subjected to work-hardening processes when the second blank sheet is press-formed into the half-pressed panel and the half-pressed panel is forced against the press-formed panel. The work-hardened die panel and punch panel have an increased surface hardening and hence they are highly resistant to wear.
The elastic material comprises urethane rubber. When the die panel or the punch panel has a rectilinear portion extending parallel to the axis of relative movement the master die and punch, the elastic material is preferably elastically deformable to an extent more than twice of the thickness of the second blank sheet. By using such elastic material, it is possible to produce a pressure or force distributed uniformly over the entire surface of the half-pressed panel while the half-pressed panel is forced against the press-formed panel.
The back-up material may be a high-strength cement or synthetic resin. When the high-strength cement is used, the applying step includes placing a slurry of high-strength cement on the rear surface of the die panel or the punch panel while removing babbles from the slurry by the action of a vacuum. It is preferable that the applying step further includes applying vibrations to the slurry to promote separation of the babbles from the slurry.
The above and other objects, features and advantages of the present invention will become manifest to those versed in the art upon making reference to the following description and accompanying sheets of drawings in which certain preferred structural embodiments incorporating the principle of the invention are shown by way of illustrative example.